1. Field of the Invention
The present invention relates to a metallic bipolar plate for a fuel cell, and more specifically for a proton-exchange membrane fuel cell (PEMFC). According to the invention, this plate has a coating comprising a compound capable of absorbing metal cations, advantageously in the form of an ionomer, for example, of perfluorosulphonic acid (PFSA) polymer type.
A cell provided with such a plate, especially usable to generate electric power, has a better corrosion resistance and an improved performance, as well as an increased lifetime. This type of cells may especially be envisaged as a power source for motor vehicles produced at a large scale in the future.
2. Description of Related Art
A fuel cell is an electrochemical device which converts chemical energy directly into electrical energy. A fuel cell comprises a stack of several series-connected cells. Each cell generates a voltage in the order of 0.7 volt and their stacking enables to generate a power supply voltage of higher level, for example, in the order of some hundred volts.
More specifically, a proton exchange membrane fuel cell or PEMFC comprises at least one unit cell containing a membrane-electrode assembly or MEA, formed of an electrolytic membrane selectively letting through protons and, on either side of this membrane, the anode and the cathode.
The membrane is generally made of a perfluorosulphonic ionomer (PFSA), such as Nafion®. The electrodes, also called catalytic or active layers, contain a catalyst, advantageously platinum (Pt), supported by carbon, and possibly an ionomer, generally the same as that forming the membrane.
At the anode level, dihydrogen (H2), used as fuel, is ionized to produce protons crossing the membrane. The electrons produced by this reaction migrate towards a flow plate and then cross an electric circuit external to the cell to form an electric current. At the cathode level, oxygen (O2) is reduced and reacts with the protons having crossed the membrane to form water.
Gas diffusion layers or GDL, conventionally made of graphite fibers, are interposed between the electrodes and the flow plates.
As already mentioned, a PEMFC may comprise a stack of unit cells, and thus a plurality of MEAs arranged between two flow plates. In this configuration, two adjacent unit cells are generally separated by a same plate, having one of its surfaces in contact with the cathode compartment of a first MEA and its other surface in contact with the anode compartment of a second MEA (FIG. 1). For this reason, the flow plates of PEMFCs are generally called “bipolar plates”.
Thus, a bipolar plate performs several functions, including, in particular:                the distribution of gases (H2, O2, or air) and the discharge of the water formed, possibly via channels and/or ports formed therein;        the collection of the electrons generated at the anodes of the different unit cells, which implies for the plate to be electrically conductive;        the cooling of the unit cells, possibly via the flowing of a cooling liquid therethrough;        a mechanical support for the MEA.        
For cost, bulk, and performance reasons, a bipolar plate for PEMFC is generally made of metal.
The bipolar plate is however placed in a corrosive environment, which may cause its degradation along time with, as a consequence, the salting out of metal ions in the case of a metallic plate.
Such a corrosion may have several consequences:
In extreme cases, corrosion phenomena may result in a perforation of the plate, causing a loss of tightness between the anode and cathode compartments.
The plate corrosion may cause the forming of an insulating oxide layer on the metal, which increases its resistance of contact with the gas diffusion layer, whereby the electric resistance between the plate and the GDL is increased, and the fuel cell performance is decreased.
Further, the salted-out metal ions may alter the operation of the electrolytic membrane. Thus, WANG et al. (Journal of Power Sources 183, 2008, 576-580) and KELLY et al. (Journal of Power Sources 145, 2005, 249-252) have highlighted the adverse influence of metal ions (Fe3+ and Cr3+; Fe3+, Cu2+ and Ni2+, respectively) on the conductivity of a Nafion®-type proton exchange membrane. The metal ions may also “contaminate” the catalyst present at the electrodes, by occupying the active sites, and thus decrease the catalytic activity. Accordingly, the cell performance and durability may be very negatively affected by the degradation of the bipolar plate.
To decrease the corrosion of metallic plates, solutions implemented in prior art have especially comprised protecting said plates by means of a coating also having electric conduction properties.
It may in particular be a coating containing noble metals, or carbon in a polymer matrix.
On this regard, document EP 2469634 describes a metallic bipolar plate having a discontinuous film containing a noble metal (Au, Pt, Ir, Ru) and an oxide film formed on the plate portion which does not comprise the discontinuous film.
Document EP 2112250 describes a metallic bipolar plate having, at its surface, a passive layer containing chromium, and then a film made of nitride metal (MNx), metal/nitride metal (M/MNx), carbide metal (MCy), or boride metal (MBz).
Further, document U.S. Pat. No. 7,699,916 describes a bipolar plate having a polymer film coating containing conductive particles smaller than 10 micrometers.
Document FR 2971628 describes the insertion of layers made of a polyurethane matrix having conductive material fillers included therein, between the metallic bipolar plate and the gas diffusion layer.
In practice, coatings developed in prior art enable to physically protect the metallic bipolar plate from the corrosive environment of the fuel cell.
There however is a permanent need to develop new technical solutions enabling to limit the degradation of metallic plates for PEMFCs and above all to improve the durability of the performance of such cells.